1. Field of the Invention
This invention relates generally to a hingeable derrick, and more particularly to derrick for use on an offshore drilling rig where the derrick may be easily folded to allow passage of the rig under a bridge or to increase stability during severe weather and may be easily returned to an upright position for use.
2. Description of the Related Art
Offshore drilling structures typically comprise a four sided derrick structure. Derricks are usually designed for one time assembly on offshore drilling rigs, such as jack ups, semi submersible, and drill ships. In some instances, three sided cantilever type masts have been used in place of four-sided derricks when transportation restrictions required that the structure be lowered. The lowering was done to provide necessary height clearance and/or increased stability during tow in severe weather. However, in most cases, four sided derricks are preferred over three sided masts because they provide the most efficient design. For example, they provide more capacity and stiffness with less weight.
Based on the foregoing, it would be desirable to provide a four sided derrick that can be folded and thus provide reduced height for clearance and stability and may be easily returned to an upright position for use.